A technology for forming a monochromatic or color image has been known, in which an ink sheet having an ink layer containing a thermally difusible dye faced to an image receiving layer of image reciving sheet, and the ink sheet is imagewise heated by a thermal head or a laser light beam so as to transfer the dye to form an image on the image reciving layer. Such the image forming method has a fault that the sorage ability of the obtained image is insufficient when an ordinary known dye is used. As a means for improving such the fault, an image forming method using a reactive dye is proposed. In such the method, a compound such as a dye precursor, contained in the ink sheet is imagewise transferred to the image receiving sheet and reacted with a compounds so-called a dye fixing agent, contained in the image receiving sheet to form an image. For example, Japanese Patent Publication Open for Public Inspection, (JP O.P.I.) No. 9-327976, U.S. Pat. Nos. 4,880,769 and 5,534,479 propose a method in which a deprotonized cation dye is used as the dye precursor and a polymer or a oligomer of organic acid capable of protonizing the cationic dye is used as the dye fixing agent, and the cationic dye is re-protonized by thermal transfer to form an image. In JP O.P.I. No. 5-221151 proposes a method for forming an image in which a specific dye having a reactive group is used as the dye precursor and a compound having active hydrogen is used as the dye fixing agent and an image is formed by reaction of them by thermal transfer. Furthermore, JP O.P.I. Nos. 59-78893, 59-109394 and 60-2398 disclose a method for forming an image in which a thermally diffusible dye capable of chelating, hereinafter referred to a post-chelating dye, as the dye precursor and a metal ion-containing compound, hereinafter referred to a metal source, is used as the dye fixing agent, and a metal chelate is formed by reaction of them by thermal transfer to form an image.
When the image is formed by using such the reactive dye, a higher stability of the fixed dye can be obtained by raising the reaction efficiency of the dye precursor and the fixing agent. Accordingly, the storage ability of image can be raised by subjecting the image formed by the dye to a re-heating treatment. For example, JP O.P.I. No. 4-89292 proposes a method for raising the chelating rate in which an image formed by transfer of the post-chelating dye is further re-heated. In JP O.P.I. No. 10-138646, a method is proposed in which an image formed by reaction of a specific dye with a compound having active hydrogen is re-heated.
Methods using a heating roller, a heating press, heating blower and infrared rays, as a means for re-heating treatment, are disclosed in JP O.P.I. No. 4-29890 and 4-52223. However, such the methods are not suitable for practical use since the device for re-heating is installed in the thermal transfer recording apparatus. As a result, such problems are caused that the apparatus size is made larger, the safety is lowered and the coat is raised. A re-heating apparatus may be separately installed but the handling conveniency is lowered since two steps of operation is necessary to form an image. When a contact heating means such as a heating roller and a heating press is used, it is necessary to insert a protecting sheet between the heating means and the receiving sheet to prevent the contamination caused by transfer of the dye to the heating means. The use of the protecting sheet causes lowering in the handling conveniency and increasing in the cost. On the other hand, a method using a thermal head as the heating means has been proposed. For example, Japanese Examined Patent Publication No. 4-55870 discloses a method using a sublimation dye transfer paper having a frame of non coated area is sequentially provided with a frame of dye coated area and the image receiving sheet is re-heated by a thermal head through the non coated area after the transfer of image. Such the method is advantageous in the cost and handling since the re-heating treatment can be performed in the thermal transfer recording apparatus.
However, when an image is formed by using the reactive type dye, a problem is caused that the dye is inversely transferred to the protecting sheet or the non coated area of the ink sheet at the time of the re-heating since the dye is not sufficiently reacted in the transferred image area or the reacted dye is existed near the surface of the image receiving sheet.
When the inverse transfer of the dye is occurred, the density and the chroma of the image are lowered. It is necessary to transfer an excessive amount of the dye to compensate the shortened dye. The excessive transfer of the dye is useless from the viewpoint of cost. Furthermore, it is necessary to apply a higher strength of energy to excessively transfer the dye. Accordingly, the load of the thermal head is increased and the adhesion by fusion of the ink sheet with the image receiving sheet and the sticking of the back surface of the ink sheet are probably occurred. An unevenness of the image is formed after re-heating when the inverse transfer is not uniformly occurred.